Impact of built-in fields and contact configuration on the characteristics of ultra-thin GaAs solar cells

We discuss the effects of built-in fields and contact configuration on the photovoltaic characteristics of ultrathin GaAs solar cells. The investigation is based on advanced quantum-kinetic simulations reaching beyond the standard semi-classical bulk picture concerning the consideration of charge carrier states and dynamics in complex potential profiles. The thickness dependence of dark and photocurrent in the ultra-scaled regime is related to the corresponding variation of both, the built-in electric fields and associated modification of the density of states, and the optical intensity in the films. Losses in open-circuit voltage and short-circuit current due to leakage of electronically and optically injected carriers at minority carrier contacts are investigated for different contact configurations including electron and hole blocking barrier layers. The microscopic picture of leakage currents is connected to the effect of finite surface recombination velocities in the semi-classical description, and the impact of these non-classical contact regions on carrier generation and extraction is analyzed.Comment: 5 pages, 8 figure

Physical activity engagement in people with Multiple Sclerosis

There is a growing body of evidence confirming the benefits of physical activity and exercise in people with Multiple Sclerosis (MS). However, most engage in low levels of physical activity when compared to the general population and other chronic illness groups. Despite this, there is a paucity of research aiming to better understand physical activity behaviour and exploring factors potentially influencing physical activity engagement in this population. The key aims of this research were therefore to gain a more in-depth understanding of the barriers and facilitators to physical activity from the perspective of people with MS, and use this information to inform the development of a novel approach to activity engagement for people with MS: The Facilitating Activity for well-Being (FAB) Programme. However, it became clear early on that prior to undertaking this work it would be necessary to carry out some preliminary research exploring physical activity measurement for this population. As a result, three phases make up this doctoral work, in which six research projects were carried out. Phase I of this PhD focused on physical activity measurement in people with MS and included three studies exploring both self-report and objective measurement. Phase II focused on exploring the barriers and facilitators to physical activity engagement for people with MS and incorporated two studies: a questionnaire study and a qualitative inquiry. Phase III was the final phase of this work and involved developing The FAB Programme, informed by existing literature and findings from earlier phases of this research. This phase also included the final study conducted as a part of this doctorate where the FAB Programme was piloted in people with MS and revised in response to feedback from participants and their clinicians. Several findings from this work challenge and/or augment existing knowledge in this field. In Phase I, a key contribution was the development of a conceptually sound self-report measure of physical activity for people with MS (the PADS-R). In contrast, despite their intuitive appeal, Actical accelerometers did not appear to be a valid and reliable objective measure of physical activity in people with MS; challenging assumptions about the accuracy of these devices in populations with chronic disabling conditions. A number of novel findings were determined in Phase II, including: a) that beliefs and perceptions about physical activity appear to play an important role in the decision to engage in physical activity for people with MS; b) the decision to engage in physical activity is complex, fluid and individual; challenging the notion of intention-behaviour continuity (a basic assumption of many existing theories of behaviour change; and c) that health professionals appear to play an important role in shaping physical activity behaviour in people with MS. A key contribution of Phase III to the field was the development of a theoretically driven, evidence based approach to activity engagement for people with MS – The FAB Programme. This programme was found to be acceptable to both people with MS and their clinicians and perceived benefits reported by participants indicated it has the potential to effectively facilitate physical activity engagement in this comparatively sedentary population. Findings also indicated that engagement of clinicians in a new way of working is challenging and something to be grappled with due to the paradigm shift required and perceived threats to their scope of practice. The body of work presented here has contributed significantly to the field of physical activity engagement for people with MS in terms of conceptual clarity, measure development, improved understanding of the complexity regarding physical activity engagement and through the development of a novel approach to activity engagement for people with MS. The development of an intervention, such as the FAB Programme, which facilitates people with MS to engage in physical activity should result in considerable health gains, and is likely to enhance the positive effects achievable in more traditional exercise programmes due to the expectation that adherence will improve

Comparison of the device physics principles of planar and radial p-n junction nanorod solar cells

A device physics model has been developed for radial p-n junction nanorod solar cells, in which densely packed nanorods, each having a p-n junction in the radial direction, are oriented with the rod axis parallel to the incident light direction. High-aspect-ratio (length/diameter) nanorods allow the use of a sufficient thickness of material to obtain good optical absorption while simultaneously providing short collection lengths for excited carriers in a direction normal to the light absorption. The short collection lengths facilitate the efficient collection of photogenerated carriers in materials with low minority-carrier diffusion lengths. The modeling indicates that the design of the radial p-n junction nanorod device should provide large improvements in efficiency relative to a conventional planar geometry p-n junction solar cell, provided that two conditions are satisfied: (1) In a planar solar cell made from the same absorber material, the diffusion length of minority carriers must be too low to allow for extraction of most of the light-generated carriers in the absorber thickness needed to obtain full light absorption. (2) The rate of carrier recombination in the depletion region must not be too large (for silicon this means that the carrier lifetimes in the depletion region must be longer than ~10 ns). If only condition (1) is satisfied, the modeling indicates that the radial cell design will offer only modest improvements in efficiency relative to a conventional planar cell design. Application to Si and GaAs nanorod solar cells is also discussed in detail

The Social World of Content Abusers in Community Question Answering

Community-based question answering platforms can be rich sources of information on a variety of specialized topics, from finance to cooking. The usefulness of such platforms depends heavily on user contributions (questions and answers), but also on respecting the community rules. As a crowd-sourced service, such platforms rely on their users for monitoring and flagging content that violates community rules. Common wisdom is to eliminate the users who receive many flags. Our analysis of a year of traces from a mature Q&A site shows that the number of flags does not tell the full story: on one hand, users with many flags may still contribute positively to the community. On the other hand, users who never get flagged are found to violate community rules and get their accounts suspended. This analysis, however, also shows that abusive users are betrayed by their network properties: we find strong evidence of homophilous behavior and use this finding to detect abusive users who go under the community radar. Based on our empirical observations, we build a classifier that is able to detect abusive users with an accuracy as high as 83%.Comment: Published in the proceedings of the 24th International World Wide Web Conference (WWW 2015

Cultures in Community Question Answering

CQA services are collaborative platforms where users ask and answer questions. We investigate the influence of national culture on people's online questioning and answering behavior. For this, we analyzed a sample of 200 thousand users in Yahoo Answers from 67 countries. We measure empirically a set of cultural metrics defined in Geert Hofstede's cultural dimensions and Robert Levine's Pace of Life and show that behavioral cultural differences exist in community question answering platforms. We find that national cultures differ in Yahoo Answers along a number of dimensions such as temporal predictability of activities, contribution-related behavioral patterns, privacy concerns, and power inequality.Comment: Published in the proceedings of the 26th ACM Conference on Hypertext and Social Media (HT'15